1. Field of the Invention
The present invention relates to a position detecting apparatus, and more particularly to an apparatus which generates a high-accuracy position-detective signal by the interpolation of a plurality of detecting signals from a plurality of detection elements.
2. Description of the Related Art
Various position detecting apparatuses have been proposed and in practical use. FIGS. 10 and 11 show a typical position detecting apparatus which detects a position of movable objects. This conventional position detecting apparatus is provided with a measuring scale 901 and a MR sensor 902 which is constituted by three units of magnetic resistance elements. Three-phase signals outputted from the three magnetic resistance elements of the MR sensor 902 are supplied to sampling circuits 906, 907 and 908 through buffer amplifiers 903, 904 and 905, respectively. The sampling circuits 906, 907 and 908 are turned on sequentially so as to supply the analog signals from the buffer amplifiers 903, 904 and 905 to an AD conversion circuit 909 sequentially. The AD conversion circuit 909 converts the analog signals to digital signals and supplies them to a central processing unit (CPU) 910. The CPU 910 executes a calculating operation for obtaining a position of a movable object as to each of the three-phase signals A, B and C.
Next, the manner of this calculating operation will be discussed hereinafter. As shown in FIG. 7, each phase of the three-phase signals A, B and C is shifted from those of the other signals A, B and C by 120.degree.. Assuming that the signal A is standard, the three-phase signals A, B and C can be represented by the following equations A=G.sub.A sin.theta., B=G.sub.B sin{.theta.+(2.pi./3)}, C=G.sub.C sin{.theta.-(2.pi./3)}. A cycle period of the signal A can be divided into first to sixth intervals. In the first interval, the amplitude of the signals A, B and C are A&gt;C&gt;B. In the second interval, A&gt;B&gt;C. In the third interval, B&gt;A&gt;C. In the fourth interval, B&gt;C&gt;A. In the fifth interval, C&gt;B&gt;A. In the sixth interval, C&gt;A&gt;B. Therefore, it is possible to distinguish which interval is positioned by the detected signals from the amplitude relationship of the signals A, B and C. When a straight line is drawn on points at each of where two signals are the same in the amplitude, a straight line extending to fight and lower direction is drawn in the first, third and fifth intervals, and a straight line extending to right and upper direction is drawn in the second, fourth and sixth intervals in FIG. 7. FIG. 8 shows an enlarged view of the second interval of FIG. 7. In case that the amplitude of the signals A, B and C is large, the gradient of the line becomes large as shown by a real line in FIG. 8. In case that the amplitude of the signals is small, the gradient of the line becomes small as shown by a broken line in FIG. 8. Therefore, in case of a large gradient, the offset of the phase of the signal is detected as if it takes a small value at a point a with respect to the detection signal v in FIG. 8. Further, in case of small gradient, the offset of the phase of the signal is detected as if it take a large value at a point b in FIG. 8. Therefore, it is difficult to execute a accurate position detection according to the offset amount of the phase of the signal in the above-mentioned situation. As shown in FIG. 9, by multiplying a multiplying factor (V.sub.max -V.sub.min)/(V.sub.1max -V.sub.1min) with the signal v1 so that the amplitude (V.sub.1max -V.sub.1min) is changed to the amplitude (V.sub.max -V.sub.min), the offset amount b of the signal v1 becomes equal to that of the signal v2.
On the basis of this concept, the conventional position detecting apparatus has been arranged so as to detect a maximum value V.sub.max and a minimum value V.sub.min of the intersections between the phase signals and to obtain a center voltage (V.sub.max +V.sub.min)/2, for executing a correction by an offset voltage. Next, the convention position detecting apparatus is arranged to obtain a multiplying factor functioning as a reference amplitude by the calculation of (V.sub.max -V.sub.min) and to correct the signals A, B and C by multiplying the obtained multiplying factor with the detection signal, in order to execute a position detection by means of the corrected signal. Accordingly, this conventional position detecting apparatus is required to detect the previous maximum value V.sub.max and minimum value V.sub.min of the intersections since this apparatus executes a position detection by detecting an actual amount of the phase offset according to a ratio (V-V.sub.min)/(V.sub.max -V.sub.min) wherein V is a present detected voltage, (V-V.sub.min) is a present deviation, and (V.sub.max -V.sub.min) is a maximum deviation.
However, it may happen that the gain level at a turn-on time of an electric source becomes different from the gain lever at a turn-off time due to the change of the position of the measuring scale or deviation of the output of the electric source. If such difference of the gain lever is occurred in the event that the electric source is turned on after the turn-off of it, the accurate detection by this apparatus may be avoided. Further, if this conventional position detecting apparatus employing gage system shown in FIG. 11 is applied to a machine tool or industrial equipment, the position of the measuring scale may further frequently generates a difference between a turn-on time and a turn-off time of the electric source. This generates an error and avoids an accurate position detecting operation of this apparatus.